In a conventional illumination device (lamp) used in outdoor areas, radiation fins are usually not employed because they deteriorate water drainage or become a cause of dirt accumulation (see, e.g., JP2007-242258A, claim 1, FIGS. 1, 3 and 4, paragraphs 0012, 0013 and 0015).
However, the conventional illumination device without radiation fins is problematic in heat radiation and, therefore, the lifespan of a lamp such as an LED or the like becomes short. If radiation fins are employed as a solution to these problems, water or other material is gathered in the grooves between the radiation fins. This poses a problem in that the illumination device undergoes corrosion.